Conformations of Unsaturated, Cyclic Sugar Derivatives

Conformations of Unsaturated, Cyclic Sugar Derivatives 

When four of the atoms of the pyranoid ring are constrained to coplanarity, as by a double bond or an epoxide ring, two half-chair forms are possible, one of which will usually be favored. When five 

The first conformational assignment for an unsaturated carbohydrate derivative was made from an analysis of the n.m.r. spectrum of 3,4,6-tri-0-acetyl-l,5-anhydro-D-hex-l-enitol (tri-O-acetyl-D-glucal) in chloroform solution. It was shown that this 1,2-unsaturated, pyranoid 

From a more extensive n.m.r.-spectral study of various acylated, 2,3-un saturated aldose derivatives, it was further observed that the anomeric effect is a dominant factor in determining the conformations of these derivatives. Thus, tetra-O-acetyl-3-deoxy-a-D-cryfhro-hex-2-enopyranose (39) adopts the f/ conformation, whereas the corresponding /3-D anomer (40) exists in the alternative conformation 

Consideration of the anomeric equilibria of various acetylated, 2,3-un saturated, pyranoid sugars led to the conclusion that polar, allylic substituents favor the quasi-axial orientation (0.8 kcal.mole for the acetoxyl group). Such an allylic effect was found to exert significant control over the conformational tendency and configurational equilibria of unsaturated, pyranoid sugar derivatives. Thus, tri-O-acetyl-2-hydroxy-D-xylal (41) adopts the H (d) conformation. In 

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VIII. Conformations of Unsaturated, Cyclic Sugar Derivatives.122... 

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